feat: make copy command feature to be available only for macOS.

[kyu08]

Currently, we have no resource to support linux for this feature.
ref: #577 (comment)

TODO

• make copy command feature to be available only for macOS
• update document

[BaccanoMob]

I get a feeling you will hard code pbcopy if its macOS. Wont it be easier to have a env for it specified in the .bashrc or .zshrc file to make it generic?

FZF_MAKE_COPY="xclip -selection clipboard" # x11
FZF_MAKE_COPY="wl-copy" # wayland
# below should also be possible
FZF_MAKE_COPY="pbcopy" # osx

I remember fzf-make does not have a config yet. So env is the best alternative I can think of. Plus this would close #577.

Also I found that, copy command works fine when I use desktop environment but when I used in tty, copy command didnt work. I got Failed to copy command to clipboard: Unknown error when interacting with the clipboard: x11 server. So it would be ideal if the FZF_MAKE_COPY executes no matter what afterwards if its specified so the command goes to clipboard or give it priority if it exists.